Multiple Quantum Well Structure and Light Emitting Diodes

ABSTRACT

A light emitting diode has a light emitting region including a multiple quantum well structure, including a first protection layer, a first intermediate layer over the first protection layer, a quantum barrier layer over the first intermediate layer, a second intermediate layer over the well layer, a second protection layer over the second intermediate layer, and a quantum barrier layer over the second protection layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority to,PCT/CN2014/094873 filed on Dec. 25, 2014, which claims priority toChinese Patent Application No. 201410110224.8 filed on Mar. 24, 2014.The disclosures of these applications are hereby incorporated byreference in their entirety.

BACKGROUND

Light-emitting diodes (LEDs), due to their high luminance and energyefficiency, have been recognized as the third generation lighting sourceand seen rigorous development in recent years. A GaN-based epitaxialwafer grown on a substrate can be a foundation for typical LEDs anddetermines their performance.

In general, a LED epitaxial wafer comprises a substrate, an N-typeconductive layer, a stress releasing layer, a light emitting layer, anelectron blocking layer, a P-type conductive layer and a P-type contactlayer, wherein, the structure of the light emitting layer and thequality of the crystal are determinant to the photoelectric propertiesof the semiconductor device. However, in semiconductor devices based onGroup-III nitrides, as the quantum well layer mostly differs from theN-type conductive layer and the quantum barrier layer in terms ofmaterials and components, stress will be generated in the quantum welllayer, thus generating polarization charges at the interface between thequantum well and the quantum barrier to form a polarization field. Thispolarization field will cause quantum Stark effect in the quantum welllayer that makes electrons and hole wave functions separated, thusreducing photoelectric conversion efficiency and light emittingefficiency.

SUMMARY

The inventors of the present disclosure have recognized that the lightemitting efficiency of the quantum well has become a bottleneck forimproving the Group-III semiconductor device performance. Therefore, itbecomes a research focus to reduce polarization charges and the quantumStark effect in the quantum well and to improve the light emittingefficiency of devices. In the Chinese patent CN 1552104A, a method forreducing polarization charges in the quantum well has been disclosed, inwhich, an In_(x)Ga_(1-x)N/In_(y)Ga_(1-y)N superlattice structure isinserted before growth of the quantum well to release the quantum wellstress, thus reducing the polarization charges in the quantum well andimproving light emitting efficiency. According to CN 102760808A, a lightemitting region structure is disclosed, in which, the quantum barrierlayer is divided into three layers. The layers at the two sides are madeof GaN and the middle layer is made of AlInGaN to release stress in thequantum barrier layer. However, the above technique fails to completelyrelease the quantum well stress. Chinese patent CN 102449737A disclosesa method to reduce the polarization charges in the quantum well bygrowing a Group-III nitride film on a non-polar or semi-polar surface.However, in this method, only few quantum well polarization charges arefound in the non-polar plane or the semi-polar plane, with a smallpolarization field, and the quantum well energy band inclination isreduced; therefore, more In components are required to achieve the samewavelength. As it may be needed to lower the growth temperature in orderto improve the efficiency in the In incorporation in the non-polar orsemi-polar surface, crystal quality of the quantum well will beadversely impacted. Therefore, the inventors of the present disclosurehave recognized a need to further reduce the polarization charges in thequantum well layer.

The present disclosure provides a light emitting diode structure and thefabrication method thereof, in which. A light emitting region based onGroup-III nitrides is employed to reduce the polarization charge effectin the quantum well layer.

The light emitting diode structure disclosed in the present inventioncomprises, from bottom to top, a substrate, a buffer layer, an N-typeconductive layer, a stress releasing layer, a light emitting region, anelectron blocking layer, a P-type conductive layer and a P-type contactlayer. The N-type conductive layer is made of n-type doped Group-IIInitrides; and the P-type conductive layer is made of p-type dopedGroup-III nitrides. The light emitting region has at least one quantumwell structure, comprising: a first protection layer based on Group-IIInitrides, a first intermediate layer based on Group-III nitrides on thefirst protection layer, a quantum well layer based on Group-III nitrideson the first intermediate layer, a second intermediate layer based onGroup-III nitrides on the quantum well layer, a second protection layerbased on Group-III nitrides on the second intermediate layer, and abarrier layer based on Group-III nitrides on the second protectionlayer.

In the aforesaid quantum well structure, an intermediate layer based onGroup-III nitrides grown in variable temperatures is inserted betweenthe well layer and the low-temperature protection layer to effectivelyreduce the polarization charges in the quantum well structure, reducequantum Stark effect and improve quantum efficiency. This intermediatelayer can also eliminate the additional barrier caused by silicon in thequantum barrier layer, reduce working voltage and improve photoelectricconversion efficiency.

In some embodiments of the present disclosure, the light emitting regionof the light emitting diode comprises 2-20 repetitions of the aforesaidquantum well structures, wherein, the growth conditions for eachrepeated quantum well structure can be the same or different. Forexample, in 20 pairs of quantum well structures, the periodicthicknesses and growth temperatures of the first 10 pairs can be lowerthan those of the other 10 pairs.

In some embodiments, the first protection layer is made ofAl_(a)In_(b)Ga_(1-a-b)N (0≦a≦1, 0≦b≦1, 0≦a+b≦1), whose forbidden bandwidth should be larger than that of the quantum well materials. Thecomposition of this layer can be set based on that of the well layer tominimize the polarization field of the latter. The thickness is 0-5 nm,and the growth environment is pure N₂ or H₂ or their combination. Thegrowth temperature is no lower than that of the well layer, andpreferably, no higher than the well layer temperature by 100° C.

In some embodiments, the first intermediate layer is made ofAl_(p)In_(q)Ga_(1-p-q)N (0≦p≦1, 0≦q≦1, 0≦p+q≦1). The composition willgradually change during growth. The forbidden band width is no largerthan that of the first protection layer and no lower than that of thequantum well layer, and presents gradual decrease during growth. Thethickness is 0-5 nm, and the growth environment is pure N₂ or H₂ ortheir combination. During growth, the temperature is reduced from thegrowth temperature of the first protection layer to the quantum welllayer temperature. Based on component distribution, the correspondingcooling method is adopted, e.g., linear cooling or secondary curvecooling.

In some embodiments, the quantum well layer is made ofAl_(x)In_(y)Ga_(1-x-y)N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1), in which, thecomponents can remain unchanged or change. Preferably, the componentsare remained and the energy band is not changed. The quantum well layeris 0-5 nm thick, and the growth environment is pure N₂ or H₂ or theircombination. The growth temperature is 700-900° C., which can be changedor not during growth. To facilitate control of light emitting wavelengthof the device, preferably, the growth temperature of this layer remainsconstant.

In some embodiments, the second intermediate layer is made ofAl_(p)In_(q)Ga_(1-p-q)N (0≦p≦1, 0≦q≦1, 0≦p+q≦1). The componentdistribution of this layer can be same as or different from that of thefirst intermediate layer. The forbidden band width is not larger thanthat of the first protection layer and not less than that of the welllayer, and presents gradual increase during growth. The secondintermediate layer is 0-5 nm thick, and the growth environment is pureN₂ or H₂ or their combination. During growth, growth temperature isincreased from the growth temperature of the quantum well layer to thesecond protection layer temperature. Calculate the layer components onthe basis of component distribution. Based on component distribution,corresponding heating method is adopted, e.g., linear heating orsecondary curve heating.

In some embodiments, the second protection layer is made ofAl_(a)In_(b)Ga_(1-a-b)N (where 0≦a≦1, 0≦b≦1, 0≦a+b≦1), and the forbiddenband width should be larger than that of the quantum well material. Thematerial components of this layer can be set based on materialcomponents of the quantum well layer to minimize the polarization fieldof the quantum well layer. The thickness is 0-5 nm, and the growthenvironment is pure N₂ or H₂ or their combination. The growthtemperature of this layer is not lower than that of the quantum welllayer, and preferably, not higher than the quantum well layertemperature by 100° C. The component distribution, thickness and growthtemperature of this layer can be same as or different from those of thefirst protection layer.

In some embodiments, the barrier layer is made ofAl_(c)In_(d)Ga_(1-c-d)N (where 0≦c≦1, 0≦d≦1, 0≦c+d≦1), and the forbiddenband width should be larger than that of the well layer. The componentdistribution of the barrier layer materials can be changed or not. Theenergy band can be changed or not. The thickness is 0-50 nm; the growthtemperature should be not lower than the second protection layertemperature and the growth environment can be pure N₂ or H₂ or theircombination; if H₂/N₂ mixed gas is used, preferably, the H₂ amountshould change with different growth temperatures.

In some embodiments, the quantum barrier can be un-doped or be n-typedoped when it grows to the layer thickness D (D≧0); in addition,Si-doping is stopped before or when growth is ended, with dopingconcentration not more than 5×10¹⁹ cm⁻³. The doping level can be actualdoping level or average doping level. In some embodiments, doping startsafter growth of 5 nm and stops 5 nm before growth is ended with dopingconcentration of 2×10¹⁸ cm⁻³.

In some embodiments, the barrier layer has no Si doped at the wellbarrier interface to effectively reduce the barrier layer resistance andthe additional resistance at the well barrier interface. Moreover, theSi impurities are prevented from being diffused to the quantum welllayer to eliminate the pressure stress caused by Si impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of Embodiment 1.

FIG. 2 is an enlarged structure view of the light emitting region 5according to Embodiment 1.

FIG. 3 shows variant 1 of the quantum well structure according toEmbodiment 1.

FIG. 4 shows variant 2 of the quantum well structure according toEmbodiment 1.

FIG. 5 shows variant 3 of the quantum well structure according toEmbodiment 1.

FIG. 6 is a schematic diagram of Embodiment 2.

FIG. 7 is an enlarged structure view of the light emitting region 15according to Embodiment 2.

DETAILED DESCRIPTION

Detailed descriptions of the present disclosure will be given below withreference to the accompanying drawings and embodiments.

Embodiment 1

Embodiment 1 of the present disclosure will be described in detail withreference to FIGS. 1-2.

Referring to FIG. 1, the light emitting diode (LED) structure cancomprise a substrate 1, a buffer layer 2, an N-type conductive layer 3,a stress releasing layer 4, a light emitting region 5, an electronblocking layer 6, a P-type conductive layer 7 and a P-type contact layer8. Specially, the substrate 1 can be sapphire, GaN or Si substrate; thebuffer layer 2 is preferably made of GaN, AlN or AlGaN, and can be 30 nmthick; the N-type conductive layer 3 is preferably made of GaN, orAlGaN, and the Si-doping concentration is preferably 2×10¹⁹ cm⁻³; thestress releasing layer 4 is preferably made of superlattice structureswith alternative InGaN/GaN growing at 750° C. and pure N₂ environment toincrease the V-pits in the light emitting region 5 and release part ofstress in the quantum well layer; and a current spreading layer 9 can beinserted between the N-type conductive layer 3 and the stress releasinglayer 4, preferably made of AlGaN, in which, electrons pass through theN-type conductive layer 3 for lateral spreading and then flow to thelight emitting region for adding light emitting area; the light emittingregion 5 has at least one quantum well structure 10, preferably 15repeated quantum well structures, and its specific structure will bedescribed in detail with reference to FIG. 2; the P-type electronblocking layer 6 is preferably made of AlGaN growing at 750° C.-950° C.,and preferably at 800° C.; and its thickness is 50-200 nm, andpreferably 150 nm; in this layer, the blocking electrons enter to theP-type layer and combine with holes; gradient Al component growth can beadopted; doping concentrations of the P-type conductive layer 7 and theP-type contact layer 8 are preferably 1×10 cm⁻³ and 1×10²¹ cm⁻³respectively.

Referring to FIG. 2, the quantum well structure 10 comprises a firstprotection layer 10 a, a first intermediate layer 10 b, a well layer 10c, a second intermediate layer 10 d, a second protection layer 10 e anda barrier layer 10 f.

The first protection layer 10 a and the second protection layer 10 e aremade of Al_(a)In_(b)Ga_(1-a-b)N (0≦a≦1, 0≦b≦1, 0≦a+b≦1) with thicknessof 0-5 nm. Preferably, GaN is adopted (i.e., a=b=0); the thickness is 2nm; and the growth environment is pure N₂. The growth temperatures ofthese two layers should be not lower than that of the quantum welllayer, and not higher than the quantum well layer temperature by 100° C.The preferable temperature is 800° C. This layer mainly protects the Incomponents in the quantum well layer and the intermediate layer frombeing decomposed by high temperature.

The first intermediate layer 10 b and the second intermediate layer 10 dare made of Al_(p)In_(q)Ga_(1-p-q)N (0≦p≦1, 0≦q≦1, 0≦p+q≦1). Thethickness is 0-5 nm, and preferably 2 nm; and the growth environment ispure N₂, wherein, the forbidden band width of the first intermediatelayer 10 b is not larger than that of the first protection layer 10 a,and not less than that of the well layer 10 c, and presents gradualdecrease during growth; and the forbidden band width of the secondintermediate layer 10 d is not larger than that of the second protectionlayer 10 e, and not less than that of the well layer, and presentsgradual increase during growth. Preferably, during growth, the firstintermediate layer 10 b gradually changes from the energy banddistribution of the first protection layer 10 a to that of the welllayer 10 c; and the second intermediate layer 10 d changes from theenergy band distribution of the well layer 10 c to that of the secondprotection layer 10 e. During epitaxial growth, the growth temperatureof the first intermediate layer 10 b is reduced from the growthtemperature of the first protection layer 10 a to that of the well layer10 c; and the growth temperature of the second intermediate layer 10 dis increased from the growth temperature of the well layer to that ofthe second protection layer; and any cooling/heating method can be used,e.g., linear cooling/heating. The components of these two layers can becalculated based on the quantum well layer and the energy banddistributions of the first and the second protection layers to minimizethe polarization charges in the quantum well layer, thus eliminating thepolarization field of the quantum well layer, improving light emittingefficiency and reducing droop effect.

The well layer 10 c is made of Al_(x)In_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1). The thickness is 0-5 nm, and preferably 2 nm; the growthenvironment is pure N₂; the growth temperature is 700° C.-900° C., andpreferably, constant at 750° C.

The barrier layer 10 f is made of Al_(c)In_(d)Ga_(1-c-d)N (0≦c≦1, 0≦d≦1,0≦c+d≦1). The thickness is 0-50 nm, and preferably 20 nm; the growthtemperature is not lower than that of the second protection layer 10 e,and preferably 850° C. The growth environment can be H₂/N₂ mixed gas,where H₂/N₂ ratio is 0<H₂/N₂≦1. Preferably, the H₂ amount is 10 L.Introduction of H₂ can improve the lattice quality of the quantumbarrier layer and reduce dislocation and defects. The barrier layer 10 fcan be un-doped or be n-type doped when it grows to the layer thicknessD (D≧0); in addition, Si-doping is stopped before or when growth isended, with doping concentration not more than 5×10¹⁹ cm⁻³. The dopinglevel can be actual doping level or average doping level. In preferablescheme of this embodiment, doping starts after growth of 5 nm and stops5 nm before growth is ended with doping concentration of 2×10¹⁸ cm⁻³. Inthis way, the barrier layer resistance and the additional resistance atthe well barrier interface are effectively reduced. Moreover, the Siimpurities are prevented from being diffused to the quantum well layerto eliminate the pressure stress caused by Si impurities.

Variant 1

In the light emitting region 5 of the aforesaid light emitting diode,the quantum well 40 structure as shown in FIG. 3 can be used. In thisstructure, the quantum well 40 comprises: a first protection layer 40 a,a well layer 40 c, a second intermediate layer 40 d, a second protectionlayer 40 e and a barrier layer 40 f, i.e., the well layer 40 c isdirectly formed on the first protection layer 40 a.

The first protection layer 40 a is made of Al_(a)In_(b)Ga_(1-a-b)N(0≦a≦1, 0≦b≦1, 0≦a+b≦1), in which, thickness D_(a) is 0<D_(a)≦5 nmPreferably, GaN is adopted (i.e., a=b=0); the thickness is 1 nm; and thegrowth environment is pure N₂. The growth temperatures of these twolayers should be not lower than that of the quantum well layer, andpreferably, not higher than the quantum well layer temperature by 100°C. The preferable temperature is 750° C. This layer mainly protects theIn components in the quantum well layer and the intermediate layer frombeing decomposed by high temperature.

The well layer 40 c is made of Al_(x)In_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1). The thickness D_(c) is 0<D_(c)≦5 nm, and preferably 2 nm; thegrowth environment is pure N₂; the growth temperature is 700° C.-900°C., and preferably, constant at 750° C.

Variant 2

In the light emitting region 5 of the aforesaid light emitting diode,the quantum well 50 structure as shown in FIG. 4 can be used. In thisstructure, the quantum well 50 comprises a first protection layer 50 a,a first intermediate layer 50 b, a well layer 50 c, a second protectionlayer 50 e and a barrier layer 50 f, i.e., the second protection layer50 e is directly formed on the well layer 50 c.

The well layer 50 c is made of Al_(x)InyGa_(1-x-y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1). The thickness D_(c) is 0<D_(c)≦5 nm, and preferably 2 nm; thegrowth environment is pure N₂; the growth temperature is 700° C.-900°C., and preferably, constant at 750° C.

The second protection layer 50 _(e) is made of Al_(a)In_(b)Ga_(1-a-b)N(0≦a≦1, 0≦b≦1, 0≦a+b≦1), in which, thickness D_(e) is 0<D_(e)≦5 nm.Preferably, GaN is adopted (i.e., a=b=0); the thickness is 2 nm; and thegrowth environment is pure N₂. The growth temperature of this layershould be not lower than that of the quantum well layer, and preferably,not higher than the quantum well layer temperature by 100° C. Thepreferable temperature is 750° C. This layer mainly protects the Incomponents in the quantum well layer and the intermediate layer frombeing decomposed by high temperature.

Variant 3

In the light emitting region 5 of the aforesaid light emitting diode,the quantum well 60 structure as shown in FIG. 5 can be used. In thisstructure, the quantum well 60 comprises: a first protection layer 60 a,a first intermediate layer 60 b, a second intermediate layer 60 d, asecond protection layer 60 e and a barrier layer 60 f, i.e., thicknessof the well layer is 0.

The first intermediate layer 60 b and the second intermediate layer 60 dare made of Al_(p)In_(q)Ga_(1-p-q)N (0≦p≦1, 0≦q≦1, 0≦p+q≦1). Thethickness D is 0<D≦5 nm, and preferably 3 nm; and the growth environmentis pure N₂, wherein, the forbidden band width of the first intermediatelayer 60 b is not larger than that of the first protection layer 60 a,and presents gradual decrease during growth; and the forbidden bandwidth of the second intermediate layer 60 d is not larger than that ofthe second protection layer 60 e, and presents gradual increase duringgrowth. Preferably, during growth, the first intermediate layer 60 bgradually changes from the energy band distribution of the firstprotection layer 60 a to the setting distribution, and the secondintermediate layer 60 d changes from the setting distribution to theenergy band distribution of the second protection layer 60 e; thissetting distribution is determined based on the setting light emittingwavelength. During epitaxial growth, the growth temperature of the firstintermediate layer 60 b is reduced from the growth temperature of thefirst protection layer 60 a to the setting temperature; and the growthtemperature of the second intermediate layer 60 d is increased from thesetting temperature to the growth temperature of the second protectionlayer; this setting temperature is determined based on the setting lightemitting wavelength; and any cooling/heating method can be used, e.g.,linear cooling/heating. The components of these two layers can becalculated based on the setting light emitting wavelength and the energyband distributions of the first and the second protection layers tominimize the polarization charges in the quantum well layer, thuseliminating the polarization field of the quantum well layer, improvinglight emitting efficiency and reducing droop effect.

Embodiment 2

Embodiment 2 of the present disclosure will be described in detail withreference to FIG. 6. Referring to FIG. 6, the light emitting diode (LED)structure comprises a substrate 11, a buffer layer 12 on the substrate11, an N-type conductive layer 13 on the buffer layer 12, a stressreleasing layer 14 on the N-type conductive layer 13, a light emittingregion 15 on the stress releasing layer, wherein, the light emittingregion 15 is divided into 15 a and 15 b. This structure also comprises aP-type electron blocking layer 16 on the light emitting region 15, aP-type conductive layer 17 on the electron blocking layer 16 and aP-type contact layer 18 on the P-type conductive layer 17. In the abovestructure, except the light emitting region 15, structures andfabrication methods of other layers can be described with reference toEmbodiment 1.

The first part 15 a in the light emitting region 15 has 9 pairs ofperiodic repeated quantum well structures 20 and quantum well structures30. Referring to FIG. 7, the quantum well structure 20 in the lightemitting region 15 comprises a first protection layer 20 a, a firstintermediate layer 20 b, a well layer 20 c, a second intermediate layer20 d, a second protection layer 20 e and a barrier layer 20 f.

The second part 15 b of the light emitting region is on the first part15 a of the light emitting region, having 10 repeated quantum wellstructures 30. Referring to FIG. 7, the quantum well structure 30 in thelight emitting region 15 comprises a first protection layer 30 a, afirst intermediate layer 30 b, a well layer 30 c, a second intermediatelayer 30 d, a second protection layer 30 e and a barrier layer 30 f.Refer to the growth parameters of each layer in the quantum wellstructure in Embodiment 1 for the growth parameters of each layer in thequantum well structure 30.

In this embodiment, the growth conditions of the quantum well structure20 are same as those of the quantum well 30, including growth pressure,air flow, temperature, MO sources and doping sources. However, thicknessof each layer in the quantum well structure 20 is not equivalent to thatof the corresponding layer in the quantum well structure 30. In thepreferable scheme of this embodiment, thickness of each layer in thequantum well structure 20 is ⅓ of the thickness of the correspondinglayer in the quantum well structure 30. The main wavelength of the lightemitted by the quantum well structure 20 is about 400 nm, and the lightemitting efficiency of this structure is far less than that of thequantum well structure 30. The quantum well structure 20 is mainly usedfor releasing the stress of the quantum well layer in the quantum wellstructure 30 to eliminate polarization field and improve light emittingefficiency. The thinner is the barrier layer 20 f in the quantum wellstructure 20, the higher is the Si doping concentration of the barrierlayer 30 f in the quantum well structure 30, and preferableconcentration is 1×10¹⁹ cm⁻³.

All references referred to in the present disclosure are incorporated byreference in their entirety. Although specific embodiments have beendescribed above in detail, the description is merely for purposes ofillustration. It should be appreciated, therefore, that many aspectsdescribed above are not intended as required or essential elementsunless explicitly stated otherwise. Various modifications of, andequivalent acts corresponding to, the disclosed aspects of the exemplaryembodiments, in addition to those described above, can be made by aperson of ordinary skill in the art, having the benefit of the presentdisclosure, without departing from the spirit and scope of thedisclosure defined in the following claims, the scope of which is to beaccorded the broadest interpretation so as to encompass suchmodifications and equivalent structures.

1. A Group-III-nitride-based multiple quantum well structure having atleast one quantum well, comprising: a first protection layer; a firstintermediate layer over the first protection layer; a well layer overthe first intermediate layer; a second intermediate layer over thequantum well layer; a second protection layer based on Group-IIInitrides over the second intermediate layer; and a barrier layer overthe second protection layer.
 2. The multiple quantum well structure ofclaim 1, wherein: at least one of the first protection layer or thesecond protection layer is made of Al_(a)In_(b)Ga_(1-a-b)N; and 0≦a≦1,0≦b≦1, 0≦a+b≦1.
 3. The multiple quantum well structure of claim 1,wherein: thicknesses of the first protection layer and the secondprotection layer are represented with n and m, respectively; and 0≦n≦5nm, 0<m≦5 nm.
 4. The multiple quantum well structure of claim 1,wherein: a forbidden band width of the first protection layer is notless than a forbidden band width of the first intermediate layermaterial.
 5. The multiple quantum well structure of claim 1, wherein: aforbidden band width of the second protection layer is not less than aforbidden band with of the second intermediate layer.
 6. The multiplequantum well structure of claim 1, wherein: at least one of the firstintermediate layer or the second intermediate layer is made ofAl_(p)In_(q)Ga_(1-p-q)N; and 0≦p≦1, 0≦q≦1, 0≦p+q≦1.
 7. The multiplequantum well structure of claim 1, wherein: thicknesses of the firstintermediate layer and the second intermediate layer are representedwith i and j, respectively; 0≦i≦5 nm, 0≦j≦5 nm; and i and j are not 0 ata same time.
 8. The multiple quantum well structure of claim 1, wherein:during growth of the first intermediate layer, a growth temperature isreduced from a growth temperature of the first protection layer to agrowth temperature of the well layer through a monotonic decrease. 9.The multiple quantum well structure of claim 1, wherein: during growthof the second intermediate layer, a growth temperature is increased froma growth temperature of the well layer to a growth temperature of thesecond protection layer through a monotonic increase.
 10. The multiplequantum well structure of claim 1, wherein: a forbidden band width ofthe first intermediate layer is not larger than a forbidden band with ofthe first protection layer and not less than a forbidden band width ofthe well layer, and is gradually decreased during growth.
 11. Themultiple quantum well structure of claim 1, wherein: a forbidden bandwidth of the second intermediate layer is not larger than a forbiddenband width of the second protection layer and not less than a forbiddenband width of the well layer, and is gradually increased during growth.12. The multiple quantum well structure of claim 1, wherein: the welllayer is made of Al_(x)In_(y)Ga_(1-x-y)N, and 0≦x≦1, 0≦y≦1, 0≦x+y≦1. 13.The multiple quantum well structure of claim 1, wherein: the well layerhas a thickness of 0-5 nm.
 14. The multiple quantum well structure ofclaim 1, wherein: the barrier layer is made of Al_(c)In_(d)Ga_(1-c-d)N,and 0≦c≦1, 0≦d≦1, 0≦c+d≦1.
 15. The multiple-quantum well structure ofclaim 1, wherein: the barrier layer has a thickness of 0-50 nm.
 16. Themultiple quantum well structure of claim 1, wherein: the barrier layeris fully doped or partially doped with a doping concentration not morethan 5×10¹⁹ cm⁻³.
 17. A light-emitting diode (LED), comprising: anN-type conductive layer; a P-type conductive layer; and a light emittinglayer between the N-type conductive layer and the P-type conductivelayer, wherein the light emitting layer includes: aGroup-III-nitride-based multiple quantum well structure having at leastone quantum well, comprising: a first protection layer; a firstintermediate layer over the first protection layer; a well layer overthe first intermediate layer; a second intermediate layer over thequantum well layer; a second protection layer based on Group-IIInitrides over the second intermediate layer; and a barrier layer overthe second protection layer.
 18. The LED of claim 17, wherein: at leastone of the first protection layer or the second protection layer is madeof Al_(a)In_(b)Ga_(1-a-b)N; and 0≦a≦1, 0≦b≦1, 0≦a+b≦1.
 19. A method ofgrowing a Group-III-nitride-based multiple quantum well structure havingat least one quantum well, the structure comprising: a first protectionlayer; a first intermediate layer over the first protection layer; awell layer over the first intermediate layer; a second intermediatelayer over the quantum well layer; a second protection layer based onGroup-III nitrides over the second intermediate layer; and a barrierlayer over the second protection layer, wherein the method comprises:growing the first protection layer at a first temperature; growing thefirst intermediate layer at monotonically decreasing temperatures fromthe first temperature to a second temperature; and growing the welllayer at the second temperature.
 20. The method of claim 19, furthercomprising: growing the second intermediate layer with monotonicallyincreasing temperatures form the second temperature to a thirdtemperature; and growing the second protection layer at the thirdtemperature; wherein: a forbidden band width of the first intermediatelayer is not larger than a forbidden band with of the first protectionlayer and not less than a forbidden band width of the well layer, themethod further comprising gradually decreasing the forbidden bandwidthof the first intermediate layer during growth; and a forbidden bandwidth of the second intermediate layer is not larger than a forbiddenband width of the second protection layer and not less than a forbiddenband width of the well layer, the method further comprising graduallyincreasing the forbidden band width of the second intermediate layerduring growth.